The present application claims priority to Japanese Application No. P2000-081853 filed Mar. 17, 2000, which application is incorporated herein by reference to the extent permitted by law.
This invention relates to a manufacturing method of information recording media for recording various information signals such as audio signals or video image signals.
As media for recording audio, video and other various types of information, for example, optical disk recording media and magnetic disk recording media are known. As these recording media, so-called CD (Compact Disk) or DVD (Digital Versatile Disk) containing emboss pits as signal information, phase change type optical disk in which inorganic films or organic films are laminated on a groove concentrically formed and information signals are written on such films with laser beams focused from the outside, magneto-optical disk utilizing the magneto-optical effect of recording films, and in addition magnetic disks in which signals are written magnetically can be mentioned.
As a method of forming the recording layer of information signals consisting of phase pits, pregrooves and other micro convexconcaves in which data information, tracking servo signals, etc. of these recording media are recorded, for example the injection molding of plastic substrates is generally in practice. In other words, when disk-type information recording media are formed by means of an injection molding machine, a die and a stamper, information signals are transcribed from the stamper to form such information recording media.
In the meanwhile, the substrate of disk-type information recording media are generally formed by injection molding of plastics except in case of magnetic recording media of glass and aluminum. The most disadvantageous point of this substrate is that at the stage where dissolved plastic is injected into a metal cavity, the friction of the plastic being injected with the die, the pressure or temperature developing at the time of injection result in a distorted molecular orientation or a thermal distortion of plastic. The internal stress having developed within the substrate is somewhat mitigated and becomes small by the mitigation of stress in the process of cooling and solidification within the die. However, most of the internal stress having developed within the substrate is not mitigated before the solidification of plastic and remains as a residual stress within the substrate. In particular, the molecular orientation distortion resulting from the injection of plastic is hardly mitigated during the anneal and other heat processing, causing double refractions of light, warp, swell and other forms of skew.
In recent years, in order to enhance the recording density of optical disks, such means that the numerical aperture (hereinafter referred to as xe2x80x9cNAxe2x80x9d) of objectives is enlarged, or that the wavelength of laser beams is reduced to that of shortwave have been reported and put into practice. And as a result of improvements in recording density brought by those means, the depth of focus of objectives has become smaller and the tolerance for warps and swells of the substrate has become severer, and at the same time the thickness of the substrate constituting a light transmission layer has become thinner.
However, with regard to injection molding, in addition to said orientation distortion, there is a temperature gradient of 100xc2x0 C. or more between the dissolution temperature of plastic and the die temperature, and the orientation distortion grows larger as the substrate is thinner from the moment when plastic is injected into the die while the solidification process progresses, and it is extremely difficult to manufacture substrates satisfactory in all respects including capacity to transcribe, double refraction of light, warp, etc.
Also, in the field of magnetic recording media, with the rise of recrding density and the fall of prices, substrates are being developed successively, and new recording media in combination with optical disks are being developed.
In other words, according to the prior injection molding method, it is extremely difficult to produce substrates with little thermal deformation, warp or swell by transcribing minute information signals with a high precision.
Furthermore, in addition to the injection molding method, there is a method of transcribing the information signals of the stamper by heating the whole sheet or other types of substrate to its glass transition temperature or deformation temperature and by applying pressure thereon. This method can produce warps during the cooling process of the plastic material, and depending on the type of the plastic material used complex apparatuses are required resulting in a cost disadvantage.
The present invention therefore was devised in view of the past issues raised above, and it is an object of the present invention to provide a transcribing method enabling high quality transcription free of thermal deformation and warps in the manufacturing process of recording media for transcribing information signals formed on a stamper onto the substrate.
For transcribing information signals formed as micro convexconcaves on a main surface of the stamper onto the substrate, the transcribing method related to the present invention brings a main surface of the substrate and a main surface of the stamper having micro convexconcaves formed thereon into contact facing each other, pressurizes the substrate and the stamper to a prescribed pressure as they remain in contact facing each other, and raises the temperature of only the contact surface layer of the substrate which is brought into contact with the stamper to the glass transition temperature or above.
The transcribing method related to the present invention brings a main surface of the substrate and a main surface of the stamper having micro convexconcaves formed into contact to face each other, pressurizes the substrate and the stamper to a prescribed pressure as they remain in contact facing each other, and raises only the temperature of the contact surface layer of the substrate with the stamper to the glass transition temperature or above. The surface layer of the substrate which is brought into contact with the stamper begins to soften at a time when their temperature reaches the glass transition temperature of the substrate or above. And, as the substrate remains under pressure at this time, information signals formed on the stamper in the form of micro convexconcaves are transcribed on the contact surface with the stamper. In addition, the temperature of only the contact surface layer of the substrate with the stamper is high. In other words, the temperature of the whole substrate has not risen, but only a part of the substrate has a high temperature, and therefore transcription is carried out without any thermal deformation or any generation of internal stress and therefore, any deformation caused by the internal stress.
The transcribing method related to the present invention brings a main surface of the substrate and a main surface of the stamper having micro convexconcaves representing information signals formed into contact facing each other, pressurizes the substrate and the stamper to a prescribed pressure as they remain in contact, and raises the temperature of only the contact surface layer of the substrate to the glass transition temperature of the substrate or over.
In other words, as it is not necessary, according to the transcribing method related with the present invention, to heat the substrate and/or the stamper to a high temperature, neither warps, deformations, nor internal stresses of the substrate develop due to heat after the transcription of information signals and no deformations of the substrate resulting therefrom develop. And therefore, it becomes possible to transcribe with a high precision and to produce high-quality substrates. In addition, the transcribing method related with the present invention raises the temperature of only the contact surface layer of the substrate with the stamper and saves time for a cooling process of the substrate after each transcription, it is thus possible to improve production efficiency in the manufacturing process.
Therefore, according to the present invention, it is possible to provide a transcribing method of easily producing high-quality substrates free from warps or deformations.